Do not count dvr agents while creating HA ports

Presently dvr agents are not excluded when a
ha router interface is created, because of which
an interface is created even in the dvr agent namespace.

This patch fixes the issue by supporting a filter - 'agent_modes'
in the function L3AgentSchedulerDbMixin.get_l3_agents.
If this filter is defined, only the l3 agents whose modes
defined in this filter are returned

Change-Id: I9383e2a30ddc4f2c3e04ff0821726b8dc3ec89a0
Closes-bug: #1397209
Merge "Do not count dvr agents while creating HA ports"